feat(example): Add example app using channels

[greg-szabo]

Closes: #XXX

This is a simplified example application in the examples/channel folder. It's using the test crate's implementation where possible.

Things that have been done:

• fixes in app-channel (especially providing contact to the gossip-consensus actor through a channel)
• Node implementation for the example-channel app
• A simple state implementation for example-channel so it can store values.

There are a few bugs and features to fix/implement:

• The code was able to produce blocks but now it doesn't: most probably the nodes don't receive the proposer's value and prevote nil. Fix my refactor bug...
• Implement the value differently. The test crate holds a single u64 and that is just not cool.
• Load values from the file system so people can add transactions in lieu of RPC or a cli.

---

PR author checklist

• Reference GitHub issue
• Ensure PR title follows the conventional commits spec

[romac]

The PR needs a merge from main, which will require a bunch of renaming, see those relevant PRs:

• chore(code): Rename malachite-cli to malachite-test-cli #685
• chore(code): Rename malachite-actors to malachite-engine #679
• chore(code): Rename network-related crates and actors #678